In recent years, mouse and human iPS cells have been established one after another. Yamanaka et al. induced iPS cells by transferring the Oct3/4, Sox2, Klf4 and c-Myc genes into mouse fibroblasts to force the cells to express the genes [WO 2007/069666 A1; Takahashi, K. and Yamanaka, S., Cell, 126: 663-676 (2006)]. Later, it was revealed that iPS cells could also be established with 3 factors excluding the c-Myc gene [Nakagawa, M. et al., Nat. Biotechnol., 26: 101-106 (2008)]. Furthermore, Yamanaka et al. succeeded in establishing iPS cells by transferring the same 4 genes into human dermal fibroblasts [WO 2007/069666 A1; Takahashi, K. et al., Cell, 131: 861-872 (2007)]. On the other hand, Thomson and his group established human iPS cells using Nanog and Lin28 in place of Klf4 and c-Myc [WO 2008/118820 A2; Yu, J. et al., Science, 318: 1917-1920 (2007)]. Specifically, it has been shown that the number of colonies of iPS cells (establishment efficiency) is increased by adding Lin28 to the three factors consisting of Oct3/4, Sox2 and Nanog [WO 2008/118820 A2; Yu, J. et al., Science, 318: 1917-1920 (2007)]; Lin28 is described as a factor that is not indispensible for reprogramming but acts to improve reprogramming efficiency [WO 2008/118820 A2].
Known as a member of the Lin28 family is Lin28B. Lin28B was originally cloned as a gene overexpressed in hepatocellular carcinoma [Guo Y. et al., Gene, 384: 51-61 (2006)]. Lin28B, like Lin28, is an RNA-binding protein, and is known to inhibit the maturation of the microRNA let-7 to influence the differentiation and proliferation of ES cells [Lu L. et al., Eur. J. Cancer, 45(12): 2212-2218 (2009)]. Lin28B has also been suggested to mediate the progression of ovarian cancer [Lu L. et al., Eur. J. Cancer, 45(12): 2212-2218 (2009)]. However, there has been no disclosure or suggestion regarding the involvement of Lin28B in iPS cell induction.